1. Field of the Invention
The present invention relates to a differential limiting control device for controlling a differential limiting force between right and left wheels of a vehicle.
2. Description of the Related Art
Such a vehicle that is provided with a differential limiting device of an electronic control type using a control device, such as a clutch, a pump and a motor, in between right and left front or rear wheels to which the driving force of an engine is transmitted has been in practical use. In a vehicle of this kind, the differential limiting device is used to apply a differential limiting force to between the right and left wheels and to shift torque from a wheel having higher wheel speed to another wheel having lower wheel speed. Accordingly, the driving characteristic of the vehicle is properly controlled. A device that carries out the control described above is disclosed, for example, in Japanese Patent No. 2527204 (hereinafter, referred to as Document 1).
The control device disclosed in Document 1 is applied to a four-wheel drive vehicle whose base is an FR layout vehicle (Front-engine Rear-drive layout vehicle) for the purpose of preventing understeer and oversteer in a turning situation. A transfer for transferring a portion of the driving force of the rear wheels to the front wheels are arranged in between the front and rear wheels of this vehicle. Between the right and left rear wheels, there is disposed the differential limiting device that produces a differential limiting force. The control device lessens the differential limiting force of a differential limiting device and also reduces the amount of the driving force transferred to the front wheels by the transfer when it is suspected that the wheel speeds of outer wheels during turning are higher than those of inner wheels, and that the inner wheels have a grip on a road. By so doing, the device intends to prevent understeer.
The control device disclosed in Document 1 controls the differential limiting force in view of the wheel speeds of the right and left wheels. The wheel speeds of the right and left wheels, however, are no more than one of indicators indicative of the driving state of the vehicle, and they are not sufficient for proper control of the differential limiting force. For instance, even in a situation where the wheel speeds of the right and left wheels are under the same conditions, an optimum differential limiting force varies, depending on whether the vehicle's steering characteristic is understeer or oversteer. This causes the problem that the differential limiting force is improperly controlled in some driving states, and a good steering characteristic cannot be actualized.